Process and apparatus for manufacturing molded parts from granulated plastic materials

ABSTRACT

A process of manufacturing molded parts using a mold with relatively movable parts enclosing a mold cavity which is fed with plasticizable material, comprises directing grains of the plasticizable material into a vertically elongated vestibule having a lower end connected to the feed of the mold cavity and directing hot gases into the lower end of the vestibule and upwardly through the plasticizable material therein to make the material weldable and, thereafter, directing the weldable material into the cavity under pressure. The device for carrying out the process includes a feed conduit which is connected into the cavity which is disposed below a vertically elongated vestibule which has a closed top and a bottom opening into the conduit. The vestibule includes an upper substantially cylindrical first zone portion, a lower substantially cylindrical third zone portion, of a smaller diameter than the upper first zone portion, and an intermediate second zone portion interconnecting the first and third zone portions which is of frusto-conical configuration. The granular material is fed into the intermediate zone of the vestibule and hot gases are circulated to the lower end of the bottom third zone portion and upwardly through the material in each of the zones and out through the top of the first zone portion. A ram is movable in the feed conduit to direct the weldable plastic material into the mold cavity defined between the two mold parts.

This is a division of application Ser. No. 809,680 filed June 24, 1977,now U.S. Pat. No. 4,158,540.

FIELD AND BACKGROUND OF THE INVENTION

This invention relates in general to the construction and operation ofplasticizing devices and, in particular, to a new and useful process andapparatus for the manufacture of molded parts from granulated plasticswherein the granular plastic material is fed into a vertically elongatedvestibule through which hot gases are circulated upwardly through theplastic materials or after wetting the granular material with a reactivemonomeric preproduct, it is subjected to a surface polymerization andsubsequently the plastic pile becomes weldable and it is abruptlypressed into the mold cavity using a movable ram.

DESCRIPTION OF THE PRIOR ART

A process recently introduced to the trade makes it possible, owing tothe low thermal conductivity of plastics, to manufacture thick-walledmolded parts with wall thicknesses of over 10 mm economically. Accordingto this impact-sintering process, the heat which is necessary forwelding the grains of a pile required for one molded part is introducedinto the granulated raw material by a brief overheating of the grainsurfaces by means of a hot gas. After the surface layer of the grainshas been heated beyond the melting temperature, the core of the grainsremains cold and the entire pile is sintered together by abrupt pressinginto a mold cavity.

Because of the relatively small quantity of heat supplied to theindividual plastic grains, a temperature equalization will take placerapidly between their surface and core after their sintering together.In addition, the short cooling time is independent of the wall thicknessof the molded part. At shortened standing times, molded parts of lowshrinkage and improved strength result.

The manufacture of molded parts from heat-resistant plastics may occurwith the aid of this process directly in that, during the pressing, apolymerization along the grain boundaries takes place. For this purpose,the fully polymerized granulated plastic having a grain size of betweenabout 0.1 mm and 10 mm is first wetted with a reactive liquid monomer.Then a brief heating of the wetted granulated material by a hot gasfollows in which the monomer begins to wet the grain surface. At thispoint, the pile is pressed. Heat-resistance molded parts with lowshrinkage and increased strength at shortened standing times can therebybe obtained, making it possible to avoid the previously customary detourthrough the pressing of blanks and their chip-removing machining.

The speeds and pressures customary in plastics injection molding,applied in the described process for the pressing of the weldablegranulated material, prove sufficient, particularly at higher flow pathto wall thickness ratios.

It is the object of the present invention to provide an improved processby means of which dimensionally true as well as thick-walled moldedparts with uniformly dense structure and smooth surface can bemanufactured rapidly and at a high flow path to wall thickness ratio.

According to the invention, the solution of this problem ischaracterized by carrying out the pressing at speeds and/or pressurescorresponding to those in metal pressure casting. The kinetic energy,increasing with the square of the press-in speed can, together with thehigher end pressure, and despite the existing greater flow resistances,ensure the necessary densification of the molded part structure, so thatthe gas escapes from the compressed interstices and the plastic grainsweld together.

An intensive plasticizing and/or polymerization process can be initiatedmost expediently in a so-called bubble layer, into which the hot gasflowing to the vestibule under pressure also dissolves the plastic pilemetered in. The grain surfaces can thereby be freely exposed on allsides to the action of the hot gas along the long trajectories of thegrains. An additional intensification of the welding effect improved bythe increased kinetic energy is obtainable by application of theinjection stamping ("Spritzprage") method. The hollow mold, which at thebeginning of the press-in movement is still slightly open, can be closedas the mold filling comes to an end. In certain cases, it may be offurther advantage to press the weldable plastic pile into an evacuatedmold cavity.

The invention further relates to an apparatus for the practice of theprocess including a closable hollow mold, a vestibule connectable withthe mold cavity thereof, a plastics metering device for metering aplastics pile for one molded part each time into the vestibule, a hotgas supply device, and a cylinder-piston assembly for pressing theplastics pile, rendered weldable, into the mold cavity.

Apparatus has been proposed for the accomplishment of the impactsintering process introduced above. Such apparatus essentially comprisesa hollow mold, a tool vestibule, and a feeding screw with a screw headdesigned as the press piston. These parts are arranged on a frame,aligned with one another, in such a way that the tool vestibule lyingbetween the hollow mold and feed screw can be traversed by the screwhead in the direction of the hollow mold and back. The tool vestibule isformed with diametrically opposite porous wall portions, through which ahot air stream can be generated at right angles to the lift direction ofthe screw head. To eliminate the undesired plasticizing effect at thefeed screw, its housing bore is widened in the zone of the screw head.In the back wall of the hollow mold, away from the tool vestibule, acounter-ram is guided, which is also displaceable into the toolvestibule.

During the plastification of one plastics pile each time, whichpreviously passes through the feed screw into the tool vestibule,through the hot air stream, the vestibule is kept closed, on the onehand, by the screw head in the retracted position and, on the otherhand, by the counter-ram remaining in its extended position. Thecounter-ram thus separates the mold cavity from the tool vestibule inthe plastification phase.

After the grain surfaces have been heated beyond the meltingtemperature, the hot air stream is turned off and the feed screw ischased through the tool vestibule head-first as the pressing ram. Thepressing in of the plastics pile into the mold cavity occurs with thesimultaneous pushing back of the counter-ram. The cooling period beingover, the feed screw retracted and the mold opened, the counter-ramejects the molded part.

A disadvantage of this apparatus resides in that the feed screw,together with its drive as a pressing ram, which is customary in plasticinjection molding machines, is relatively slow, and the compressionpressure exerted at the end of the press-in stroke is ofteninsufficient. Another disadvantage is that a counter-ram is necessary,which during the press-in movement, exerts a braking counter-force. Thefact that the molded part is compressed essentially between the screwhead and the counter-ram permits only simple molded parts to be madewith this apparatus.

A further disadvantage is that the geometry of the tool vestibule andthe arrangement of the flow cycle for the hot air permits the formationof a turbulence layer with only a relatively narrow clearance for themovement of the single grains. The latter are therefore accessible tothe action of the hot air only in a limited degree. This may result inan incomplete plastification of the plastics pile.

SUMMARY OF THE INVENTION

The present invention provides an apparatus which overcomes thedisadvantages of the known arrangement. According to the invention, theapparatus is characterized in that, to increase the press-in speed orthe press-in pressure compared with those of a plastics injectiondevice, between the vestibule and the mold cavity, a pressing unit,similar to the shot unit of a cold-chamber pressure casting machine, isprovided. The thus constituted pressing unit with its pressing power farexceeding that of a conventional plastics injection device permits shortcycle times in the production of the molded parts, despite the ever morecomplicated molded-part and, hence, mold cavity design according tomodern requirements.

By the massive increase in speed and/or pressure, the factors ofmolded-part quality, which of course depend on these parameters, can befavorably influenced. At good dimensional accuracy and smooth surface ofthe molded parts, no density fluctuations, defects or gas inclusions areto be expected in their structure. The use of a counter-ram issuperfluous in the apparatus according to the invention.

To be able to carry out an effective plastification and/orpolymerization of the grain surfaces rapidly, initiated in the vestibuleby the hot gas supplied thereto under pressure simultaneously in thetotality of the plastics pile metered in, there is advantageouslyassociated with the filling aperture of the pressing unit, a vestibulecomposed of three zones.

A first cylindrical vestibule zone is advantageously in connection atits one end with the filling aperture of the pressing unit, at its otherend with the entrance aperture of equal diameter of a second conicalzone, and at its generated surface, with the pressure side of the hotgas supply device. The second conical zone may connect by its exitaperture having the largest diameter with the third likewise cylindricalzone of equal diameter, the latter being connected in this couplingregion with the plastics metering device and at its end away from thesecond zone with the intake side of the hot gas supply device.

It is advisable to connect the pressure side of the hot gas supplydevice to the first vestibule zone via several openings, the openingspermitting distribution on the cylinder surface thereof symmetrically onthe same level, inclined toward the jacket surface, and directed towardthe jacket surface, and directed toward the second conical zone.Expediently, the axial length of the first vestibule chamber between theopenings and the second zone is selected equal to three times thediameter of the first zone. The angle of inclination of the conical wallof the second vestibule zone portion in respect to the longitudinal axisthereof is selectable between a lower limit value of 15° and an upperlimit value of 30°, depending on the friction value of the granulatedplastic.

The measures described permit the dissolution of the plastics pilepresent in the vestibule into a bubble layer in which the surfaces ofall grains can be subjected to the plasticizing and/or polymerizationprocess simultaneously. The inner face of the second conical vestibulezone may be provided with a slippage lining to prevent adhesion of theplastics grains. The pressure cylinder of the pressing unit isadvantageously designed up to its opening into the mold cavity with aconstant inner cross-sectional area. The pressure face of the pressingunits is advantageously designed up to its opening into the mold cavitywith a constant inner cross-sectional area. The pressure face of thepressing ram movable therein can then assume after completed press-instroke the role of a mold portion so-defining the molded part.

It is advisable in many cases to provide a venting device connectingwith the mold cavity.

A simple hot gas supply device may comprise a blower, a pressure pipeleading from the pressure side thereof to the openings in the generatedsurface of the first vestibule zone, and a suction pipe extending fromthe third vestibule zone to the intake side of the blower, and a heatingdevice may be arranged in the pressure pipe, traversed by the pressuremedium and a shut-off valve.

In the event the hot gas supply device forms a closed circulationsystem, at least one bucket wheel and a down pipe extending from thelatter into the third cylindrical vestibule zone may be used as theplastics metering device. It may prove advantageous as well to arrangethe opening of the down pipe in the coupling region of the third andsecond vestibule zones tangential to the cylindrical jacket of the thirdvestibule zone.

An object of the invention is to provide a process of manufacturingmolded parts using a mold with relatively movable parts enclosing a moldcavity which is fed with plasticizable material and which comprisesdirecting the grains of plasticizable material into a verticallyelongated vestibule having its lower end connected to the feed of themold cavity, and directing hot gases into the lower end of the vestibuleand upwardly through the plasticizable material to make the materialweldable, and thereafter, directing the weldable material into thecavity under pressure.

Another object of the invention is to provide a device for manufacturingmolded parts, comprising a mold having first and second relativelymovable parts defining a mold cavity therebetween, and which includes afeed conduit connected into the cavity and has a ram displaceable in thefeed cavity for advancing the material therein and which also includes avertically elongated vestibule having a closed top and a bottom openinginto the conduit and defining on its interior and upper substantiallycylindrical first zone portion, a lower substantially cylindrical thirdzone portion of a smaller diameter than the first zone portion, and anintermediate second zone portion interconnecting the first and thirdzone portions wherein hot gases are circulated upwardly from the lowerend of the vestibule through the top thereof to weld the plasticizingmaterial and to feed it into the conduit connected into the mold cavityand which also includes a ram for directing the material in the conduitinto the mold cavity.

A further object of the invention is to provide a device for preheatingplastic granular material prior to its being delivered into a moldcavity of a plasticizing machine, which is simple in design, rugged inconstruction and economical to manufacture.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference should be had to the accompanying drawing and descriptivematter in which there is illustrated a preferred embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWING

In the Drawing:

The only FIGURE of the drawing is a schematic partial sectional view ofa plasticizing device constructed in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing in particular, the invention embodied therein,comprises a mold 1 which includes a movable mold part 12 and a fixedmold part 11. Mold 1 includes a venting device 2 including a blower 22for venting a mold cavity 13. A pressing unit 3 includes a ram 32movable in a pressing cylinder or connecting conduit 31 to advance theplasticizing material into the mold cavity. Granular plasticizingmaterial is fed from a bunker or feed funnel 61 into a vestibule,generally designated 4, where it is pretreated before it is deliveredinto the pressure cylinder and advanced into the cavity 13 by the ram32. The feed funnel 61 is part of a plastic metering device 6. Hot gasesare supplied from a hot gas supply device 5 to the vestibule 4 forpretreating the plastic material therein.

The mold 1 comprises a fixed mold half 11 and a movable mold half 12which, in the closed state of mold 1, enclose a mold cavity 13. Fixedmold half 11 is secured to fixed mold support 14. Support 14 is anchoredon a machine frame 15 and it is connected by means of columns 16 with afixed bracket (not shown) which receives a drive (also not shown) forthe closing and opening of the mold. The mold closing drive is inconnection through a drive member, such as a rod 17, with a mold support18 displaceable along the columns 16, on which mold support, the movablemold half 12 is mounted.

A vent passage 20 is provided in the fixed mold half 11 by way of whichthe mold cavity 13 can be connected by means of a vent valve 21 with avent blower 22. A remote control connection to the machine control hasbeen indicated by the control connection 23 in broken lines at the ventvalve 21.

The pressure cylinder 31 of the pressing unit 3 opens through arespective bore of the fixed mold half 11 and of the fixed mold support14 into the central region of the mold cavity 13. It has across-sectional area constant throughout, so that the pressure face ofthe pressing ram 32 movable therein forms, after completed press-instroke, a portion of the mold wall codefining the molded part. A fillingaperture 33 for the material to be pressed is arranged in the end regionof the pressure cylinder 31 away from mold 1.

The pressing ram 32 is in drive connection by a piston rod 34 with ashot piston 36 disposed in an associated hydraulic shot cylinder 35. Theoperation of the pressing unit 3 occurs through the two pressure lines37 and 38 which are connectable selectively to a pressure source or,respectively, to a tank by means of control valves, which are known perse and have not been shown.

The vestibule 4 is composed of three zones 41, 42 and 43. The outlet 411of the first cylindrical vestibule zone 41 is connected gastight to thefilling aperture 33 of the pressure cylinder 31 of the pressing unit 3.Above the outflow 411, at the cylinder jacket surface of the firstvestibule 41, several openings 412, connected with the pressure side ofthe hot gas supply device 5, are present, which openings aresymmetrically distributed on the same level, inclined toward the jacketsurface, and directed toward the second conical vestibule 42. Betweenthe openings 412 and the following second conical vestibule zone 42, thefirst vestibule zone 41 has an axial length equal to three times itsdiameter.

The angle of inclination α of the conical wall of the second vestibulezone 42 against the longitudinal axis thereof is 30°. The inner face ofthis conical wall is provided with a slippage lining.

The second conical vestibule zone 42 connects by its cross-sectionhaving the largest diameter with the third likewise cylindricalvestibule zone 43 which is of equal diameter. The latter is inconnection in this coupling region with the plastics metering device 6and, by its end away from the second vestibule zone 42, with the intakeside of the hot gas supply device 5. The hot gas supply device 5consists of a blower 51, whose pressure side is in connection via apressure pipe 52 with the openings 412 at the first vestibule zone 41. Aheating device 53 and a shut-off valve 54 are disposed in the pressurepipe 52. The control connection 55, shown in broken lines at theshut-off valve 54, indicates a remote control connection existing to themachine control.

The intake side of the blower 5 is connected by a suction pipe 55 to thethird vestibule zone 43. The plastics metering device 6 is formed by afeed funnel 61 with a bucket wheel 62 provided therebelow and a downpipe 63 extending from the latter to the third vestibule zone 43, thedown pipe 63 opening in the coupling region of the third and secondvestibule zones 43 and 42 tangentially into the cylindrical jacket ofthe third vestibule zone 43.

The apparatus illustrated in the drawing permits the following mode ofoperation:

The granulated material passing from the plastics metering device 6 intothe coupling region of the third and second vestibule zones 43 and 42slides along the conical wall of the second vestibule zone 42 toward thefirst vestibule zone 41, until it is seized by the massive hot air jetshooting upward from the latter zone. The hot air jet is produced by theblower 51 in the flow cycle formed by the vestibule 4 and the hot gassupply device 5 connected thereto, and is brought to the necessarytemperature in the heating apparatus 53. The heating apparatus 53 may bea heat exchanger or an electric heating unit traversed by air.

Across the shut-off valve 54, which was previously opened by means of aninstruction signal from the machine control (which has not been shown),and the openings 412, the hot air flows into the first cylindricalvestibule zone 41. By the ratio 1:3 of the axial length of the firstvestibule zone 41, between the openings 412 and the second vestibulezone 42, to the diameter of the first vestibule zone 41, a rate of flowcan be obtained which is two to three times the sedimentation of theplastic grains.

The balled hot air jet issuing from the first cylindrical vestibule zone41 into the conically widening second vestibule zone 42 dissolves thegranulate pile sliding down along the wall of the latter into a bubblelayer. The plastic grains are hurled up in a central stream against thecover of the third cylindrical vestibule zones 43, whereby, the grainsurface is softened by the hot medium.

The raised grains then sink along the periphery of the flow near thechamber wall back again to the cone of the second vestibule zone 42,whose slippage lining 421 prevents adhesion of the grains. The describedprocess is repeated until the grain surfaces of a plastic pile meteredinto the vestibule 4, which is necessary for the production of a moldedpart, are all plasticized, or respectively, in the case of granulatewetted with a liquid monomer, a surface polymerization is initiated. Thelatter process permits the direct production of molded parts fromheat-resistant plastics, the polymerization temperature being lower, asis known, than the plastification temperature. For the rapid metering ofthe granulated material into the vestibule 4, the plastics meteringdevice may have several bucket wheels 62.

After the plastic pile contained in vestibule 4 has been made weldable,the supply of the hot air stream is interrupted by closing the shut-offvalve 54 on the basis of a shutting instruction given from the machinecontrol. If, for example, an appropriately designed electric drive motoris chosen, the blower 51 can continue to run during the short shut-offtime. Due to the interruption of flow, the bubble layer collapses andthe plastic pipe falls through the filling aperture 33 into the pressurecylinder 31 of the pressing unit 3.

The pressing ram 32 of the latter had been previously pulled back intoits starting position by pressurization of its shot piston 36 by meansof control valves (not shown) via the pressure line 38 from the pistonrod-side pressure chamber of the shot cylinder 35. Simultaneously, theclosing of mold 1 is effected by means of the mold closing drive (notshown) connected to the rod 17.

After the filling of the weldable plastic pile into the pressurecylinder 31, the full pressure face of the shot piston 36 is alsopressurized by means of the control valves, via the pressure line 37.The differential force between the full and the piston rod-side pressureface of the shot piston 36 moves the piston together with the pressingram 32 relatively slowly against the mold cavity 13, until the rampushes the granulated material ahead of itself and closes the fillingaperture 33. At this point, the control connection 23 opens the ventingvalve 21, so that the mold cavity 13 is evacuated by the venting blower22.

After a possible brief dwell of the pressing ram 32 and subsequentpressure relief of the piston rod-side pressure chamber of the shotcylinder 35 via the pressure line 38, the weldable granulated plastic ispressed abruptly into the mold cavity 13, the venting valve 21 beingclosed by means of a closing instruction from the machine controlthrough control connection 23.

Due to the pressing unit 3, which operates at press-in velocities and/orpressure which correspond to those in metal pressure casting, theplastic grains fill the mold cavity 13 evenly and quickly and also athigh flow path-to-wall thickness ratios and weld together without airinclusions, collapsed areas and with little shrinkage. Dimensionalaccuracy, surface quality and mechanical strength can thereby beincreased.

To enhance the welding effect further, the hollow mold 1 which is stillopen a few millimeters before shot start, is closed at the end of thepress-in process.

The cooling time necessary for stripping the molded part and hence alsothe standing time of the machine are short in comparison to those inconventional injection molding and independent of the wall thickness.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A process for the manufacture of molded parts from granulated plastic, wherein the surface of the grains of the plastic pile required for one molded part is wet with a reactive monomeric preproduct and is subjected to a surface polymerization in a thin layer in a vestibule, which is connected to a mold cavity, by a hot gas supplied thereto to make it weldable with the cores of the grains remaining cool and solid and, subsequently, the surface of the grains having become weldable, the plastics pile is abrubtly pressed into the mold cavity of a hollow mold by an impact-like pressing at sufficient pressing speed and, upon termination of the compressive motion, a sufficient pressure is applied to the mold cavity, so that the plastics pile is sintered.
 2. A process for the manufacture of molded parts from granulated plastics, wherein the surface of the grains of the plastic pile is required for one molded part is plasticized in a thin layer in a vestibule, which is connected to a mold cavity by a hot gas supplied thereto, the cores of the grain remaining cold and unplasticized, and subsequently, the surface of the grains having become weldable, the plastics pile is abruptly pressed into the mold cavity of the hollow mold by an impact-like pressing and sufficient pressing speeds and, upon termination of the compressive motion, a sufficient pressure is applied to the mold cavity, so that the plastics pile is sintered.
 3. A process for the manufacture of molded parts using a mold with relatively movable mold parts enclosing a mold cavity which is fed with a plasticizable material and using a granular plastics pile required for one molded part, comprising plasticizing the surface of the grains of the plastic pile in a vestibule which is connected through the wall of a die casting unit to the mold cavity by directing hot gas therethrough with the cores of the grains remaining unplasticized, whereby the plastics pile is caused to form a bubble layer in the vestibule so as to make the surface of the grains weldable rapidly and uniformly, after the surface of the grains become weldable, roughly pressing them into the mold cavity with an impact-like pressing, and, upon termination of the compressive motion, subjecting the molding in the cavity to a pressure, the pressing speed and pressure being sufficient to sinter the grains together.
 4. A process for manufacturing molded parts, according to claim 3, wherein a bubble layer is formed and the mold parts are still slightly open at the beginning of the press-in movement and including closing the mold parts as the filling comes to an end.
 5. A process of manufacturing molded parts, according to claim 3, wherein the mold cavity is evacuated during the pressing of the weldable plastic material into the mold cavity.
 6. A process for the manufacture of molded parts using a mold with relatively movable mold parts enclosing a mold cavity which is fed with a plasticizable material and using a granular plastics pile required for one molded part, comprising wetting the surface area of the grains of the one molded part plastics pile with a reactive monomeric preproduct and subjecting the wet surface of the grains to a surface polymerization in a thin layer in a vestibule which is connected through the well of a metal die casting unit to the mold cavity by directing hot gas therethrough with the cores of the grains remaining cold and solid, whereby the plastics pile is caused to form a bubble layer in the vestibule so as to make the surface of the grains weldable rapidly and uniformly, and, after the surface of the grains becomes weldable, roughly pressing the plastics pile into the mold cavity with an impact exerted by the metal die casting unit at an impact speed and, upon termination of the compressive motion subjecting the mold cavity to a pressure, the impact speed and pressure being sufficient to sinter the grains to form the one molded part.
 7. A process of manufacturing molded parts according to claim 6, wherein the mold parts are still slightly open at the beginning of the press in movement and including the step of closing the molded parts as the filling comes to an end.
 8. A process of manufacturing molded parts, according to claim 6, wherein the mold cavity is evacuated during the pressing of the moldable plastic material into the mold cavity. 